As optical transmission filters the parallel-plate arrays of the present invention complement conventional bandgap filters, which are low-pass filters (i.e., they transmit photons having energies below the band gap of the material). However the transmission cut-off characteristics of the devices of the present invention are much sharper than those of most crystalline solids, especially in the infrared (IR) spectrum. The present invention is also superior in performance to multilayer dielectric filters because the multilayer dielectric filters are not wide-angle devices, unlike the devices of the present invention the main advantage of the present filters, unlike bandgap multilayer dielectric filters, the present filters are tunable. Fabry-Perot filters are tunable but have a very restricted field of view. In addition, the present filters are much more compact and rugged than tunable Fabry-Perot filters.
The only other work known to the inventors on optical filters using micromachined structures is that described by MicroParts, a company based in Germany, in a 1991 brochure, in which is described infrared bandpass and high-pass filter being developed for the Infrared Space Observatory (ISO) of the European Space Agency. These are self-supporting metal foils to be used in the ISOPHOT IR photometer. The high transparency highpass filter uses a honeycomb structure which gives stability but does not allow tuning as does the parallel-plate arrays. The MicroPart designs are effective for all polarizations of the incident radiation, which is an advantage given their design.
Some very interesting optical properties of periodic structures involve the localization of light. This is an active area of research, reviewed by Sajeev John in Physics Today, May 1991, page 32. An application of deep lamellar structures used as Bragg resonators is the concentric-circle-grating surface-emitting semiconductor lasers, which have been demonstrated previously and are reviewed in Optics Photonics News (Dec. 1992, page 41).
Some infrared (IR) emission characteristics of deep lamellar gratings have been measured in the 10 to 20 micron region by T. K. Wang and J. N. Zemel (Applied Optics, vol. 31, Feb. 1992, page 732). It was found that the emission peaks do not correlate directly from the grating equation. The inventors of the present invention suggest that among other things, using the lamellar structures as tunable filters for blackbody radiation rather than using them as the emitters themselves, as practiced by Wang and Zemel, provides far better correlation.